Linux versions 1.2.X requires the IPalias patch alias-patch-1.2.1-v1
and alias-net-tools.tar. I'm not sure if 1.3.X supports this patch yet.
For more information on the IPalias patch see
ftp://ftp.mindspring.com/users/rsanders/ipalias/

Using multiple dummy interfaces has been suggested in place of
the IPalias solution. While the dummy solution may work,
it does not appear to be as clean as the IPalias solution.
For more information on using Apache and the dummy solution
see Aram Mirzadeh's virtual hosting information at
http://www.qosina.com/apache/virtual.html

All that is required to add a new alias using the IPalias method is:
> /sbin/ifconfig eth0 alias www.domainX.com